VFD Motor Turn Down Ratio

[khanvessel]

Hi

I am working on an air cooled heat exchanger for glycol. This exchanger has 12 bays and each bay consist of two fan with motors, one motor is VFD compatable. I want to find out the turndown ratio for that motor. Min and max heat exchange required is 26,268 and 1,239 KW respectively. Max inlet temp is 91 degree C and min is 46 degree C, outlet temp is 43. Any help will be appreciated.

Thanks

 

[davidbeach]

The right motor/drive combination can take you down to zero rpm.

 

[DickDV]

The thermal limits of a motor are determined by its cooling method and the level of torque on the shaft. Since your load is a fan, the torque falls as the square of the speed. This means that at slow speeds, the torque level will be very small--mostly just friction loads.

If your motor is TEFC or ODP, the cooling is done by shaft fan and therefore falls off as shaft speed drops. It is unlikely that you have a TENV or auxiliary blower cooled motor on a fan but if you did, the cooling would be constant over the whole speed range permitting full torque output down to near-zero speed.

Assuming a TEFC or ODP motor on your fan, you can surely go down to one-tenth speed without causing thermal problems in the motor. There is little reason to go any slower as the fan doesn't move any air at such slow speeds.

 

[dpc]

As DickDV says, the limiting factor will probably be the fan curve. I suspect it will poop out well above 10% speed - more like 25-30%.

 

[waross]

If you are switching the non VFD fan and the discharge from one motor can backspin the other motor you may have some challenges. Look forr the recent thread on back spinning fans. If the air discharge paths are seperate there will be fewer issues.
respectfully

 

[khanvessel]

Thanks for yor replies.

DickDV...you are right it's a TEFC motor and actually I was worried about the heating problem with high turndown ratios. Is there a way to calculate turndown ratio?

Thanks again

 

[DickDV]

"turndown ratio" as used in the world of variable speed isthe ratio of the motor base speed to the slowest speed at which the motor is expected to operate the load.

So, for example, if you have an 1800rpm motor and will run it down to 450rpm, that's a 4/1 turndown.

Or, if you have a 3000rpm motor (50hz) and will run it down to 300rpm, that's a 10/1 turndown.

Overspeed range does not figure into turndown calculations. So, if we take the 1800rpm motor above and change the power train so it now now must run 2400rpm to 600rpm, that's a 3/1 turndown application (1800/600).

On inverter duty motors designed to operate down to zero speed, the turndown ratio becomes meaningless. Some manufacturers show 1000/1 turndown ratio for those motors but what they really mean is "good to essentially zero speed" without thermal problems.

 

[waross]

Normally, the torque/horsepower curves of a fan are such that slow speed operation is not a problem.
If the air discharge paths of the two fans are isolated from each other, you probably won't have a problem.
If the fans are dischargeing into a common plenum so that the fixed speed fan is able to apply a back pressure to the VFD fan, the torque required to maintain low flow against the backpressure may present issues not normally present.
respectfully

 

[quark]

Considering the temperature difference constant for all air flowrates, if your maximum load is 26268 kW and minimum load is 1239 kW then fan speed at low load will be 5% that of at full load. At this speed, the static pressure of the fan reduces to just 2.5%.

What are air inlet and outlet temperatures?

 

[khanvessel]

Thanks again for valuable replies

quark...Inlet air temp. is 28 C, don't know the outlet temp.